Xiaodong,
My comments are in-line
Hi Guys,
I have a question regarding the low power listening function.
I know for the receiver, we could use setLocalSleepInterval() to
invoke the periodic sleeping. And for the sender, in order to send the
packet to this receiver, setRxSleepInterval should be used to set the
receiver's sleep interval value into the packet meta data field.
*However, my question is what is the detail of this proceedure? I
don't quite understand the reason why this packet can be received by
the receiver with the same sleep interval value, as we set the
receiver's interval to the packet's meta data field.*
Basically the sender repeatedly sends the packet until it has a good
idea that the receiving node likely heard it. The sender figures out
how long to send the packet based upon the receiving node's
RxSleepInterval. A very abstract example:
Lets say the Receiving node with ID = 3 is awake for 1 second every 5
seconds. If a transmitting node (ID = 2) continually sends a packet to
Node 3 for 5 seconds, then it is very likely that the receiving node was
awake sometime during which the entire packet was transmitted.
In tinyos2.x DefaultLplP.nc, I found when to initialize the sending,
the following function will be called:
void initializeSend() {
if(call LowPowerListening.getRxSleepInterval(currentSendMsg)
> ONE_MESSAGE) {
if(call AMPacket.destination(currentSendMsg) == AM_BROADCAST_ADDR) {
call PacketAcknowledgements.noAck(currentSendMsg);
} else {
// Send it repetitively within our transmit window
call PacketAcknowledgements.requestAck(currentSendMsg);
}
call SendDoneTimer.startOneShot(
call LowPowerListening.getRxSleepInterval(currentSendMsg) + 20);
}
post send();
}
*What is the use of this SendDoneTimer?* seems to me that it schdules
next fire time at the interval +20 ms. And when the timer fires, it
will do the following:
/**
* When this timer is running, that means we're sending repeating
messages
* to a node that is receive check duty cycling.
*/
event void SendDoneTimer.fired() {
if(call SendState.getState() == S_LPL_SENDING) {
// The next time SubSend.sendDone is signaled, send is complete.
call SendState.forceState(S_LPL_CLEAN_UP);
}
}
*What does the above function mean?*
From what i can gather, there is another timer called SubSend that is
in charge of repeatedly sending the packet. This timer determines when
SubSend should stop trying to send the packet. If SubSend succeeded,
the state would not be S_LPL_SENDING, so the SendDoneTimer doesn't need
to do anything. However if it is S_LPL_SENDING, this means that the
SubSend was not able to successfully send the packet to the node,
therefore, it needs to abandon the attempt. Think of it as a watchdog
timer, if it doesn't succeed by the allotted time, the send will be aborted.
* *
**
*Another question is in the initializeSend function, when the
destination address is not broadcast, it will request ack. Does this
mean that the software ack is automatically turned on? When a receiver
receives packet, it will send out ack automatically? Is it necessary?*
There is a field in the packet that indicates this packet should be
acked. Calling PacketAcknowledgements.requestAck(currentSendMsg); Makes
the necessary modifications to the packet. The receiver will
automatically send an ACK if this bit is set in the packet upon
receiving it.
When an ACK is requested, the sendDone event for a particular packet is
not called until the ACK is received. In the case of a broadcast
packet, sendDone is called whenever the mac protocol successfully
transmits the packet (regardless of collisions, etc that may have occurred)
I hope this helps you.
-Paul
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